This invention pertains to optical collimators, and particularly to the improvement of their target illumination means.
Collimators are widely used optical instruments. They are especially valuable because they make divergent or convergent rays of light parallel. Consequently in the field of optics collimators are adapted for testing, calibrating and measuring instruments as described in U.S. Pat. Nos. 4,422,758, 4,022,533, 3,966,328, and 3,915,575, 3,518,005, and TEE Transactions on Instrument and Measurement, Vol IM-34, No. 1, March, 1985 at page 51.
In other types of collimators slits or patterns are said to be projected to infinity. Such patterns are called targets. Placing a target in the focal plane of a collimator is a known technique for making a uniform target appear to be in the far field. This far field response has led to the use of collimators in training devices and in target simulations. The technique is to use standard targets which are commonly available as target sets. Such sets include four-bar and area targets. Four-bar targets are plates having equally spaced bars or slits each generally about 0.32 cm. wide with a 7:1 aspect ratio. Area targets are square aperture plates, and the targets must be provided with slits or cut-out portions in order to be visible since their lighting emanates behind them.
When a surface is illuminated by a small light source, as is the target in a collimator, the illumination of points on the target surface off of the light axis will be lower than illumination of the area of the surface on the axis. Illumination E.sub.o of the point on the light axis is E.sub.o =I.sub.o /R.sub.o.sup.2 where I.sub.o is the normal source intensity and R.sub.o is the distance away from it.
Considering a point on the periphery of the target surface if the surface at that point was normal to light ray the illumination E(.theta.), where .theta. is the angle between the axis and the ray to the peripheral point, would be E(O)=I.sub.o cos.sup.3 .theta./R.sup.2. However, since surface at the peripheral point is not pependicular to the light ray, the intensity (E(.theta.)) is E(.theta.)=I.sub.o cos.sup.4 .theta./R.sup.2. This is known as the cosine-fourth power law.
The cosine of the angle is a fraction. Since the peripheral surface illumination is a function of the fourth power of this fraction, there is considerable reduction in illumination at the periphery of a surface. Even though the target is small, this reduction in illumination is important in a collimator. In accordance with this invention, the problem of mitigation of peripheral target of illumination in collimators is overcome.